1. Field of the Invention
The invention relates to an optically variable security attribute for documents, securities, bank notes, packaging and goods.
2. The Prior Art
The introduction of optically variable security attributes has significantly improved the safety from counterfeiting of documents, securities, bank notes, packaging and products. One kind of optically variable security attributes are holograms the data of which can be seen by human vision are subject to change when observed from different angles. The effect yielded an excellent protection against copying since angle-dependant changes could not be copied by a copying apparatus.
Such holograms conventionally consist of a support web, a layer of hardenable lacquer with diffractive structures embossed into it, as well as of a reflection and an additional protection layer. In holograms, the reflection layer consists of a highly brilliant metallization vapor deposited in a high vacuum, which for purposes of checking its authenticity may additionally contain a functional design of demetallized sites which may or may not be seen by human vision. The demetallized sites may be shaped as symbols, letters or geometric figures.
The technological equipment required for fabricating holograms as well analytical apparatus for the recognition of micro-lettering and micro-designs are nowadays readily available to counterfeiters, thus enticing increased safety against counterfeiting by further, and possibly covert, detectable attributes.
From German patent specification DE 298 07 638 it is known to provide the embossing web of a hologram in which one or more security attributes are present, with an additional individualizing characteristic which may be seen by human vision. The characteristic consists, among others, of the removal of a reflection layer, the interruptions thus created serving as the characterizing element.
German patent specification DE 40 30 493 further discloses the application, on a multi-layered data support, of an optically variable element in which there are present retro-fitted additional data recognizable by human vision, e.g. a pattern for a functional design, which are superposed on the optical effect.
German patent specification DE 39 32 505 describes diffractive structures the standard data of which may be altered, in a way detectable by human vision, by additional measures, such as, for instance, removal of the metal layer.
From German patent specifications DE 44 19 505 and DE 44 19 173 magnetizable glossy pigments and mixtures of glossy pigments have become known which are suitable for generating three-dimensional optical effects by the application of magnetic fields during or after application while the application medium is still in a liquid state. They are based upon multiply-coated platelet-shaped non-ferromagnetic metallic substrates, and they are used in lacquers and printing dyes. They are said to be capable of replacing expensive holograms; however, their yieldable color titration is still too low. German patent specification DE 195 15 988 also describes that glossy pigments are used for dying lacquers, inks and printing dyes.
Moreover, German patent specification discloses that in holograms the metallic web usually used a reflection layer may also be fabricated by vapor deposition in vacuum or by applying thin layers of aluminum. Another possibility resides in a described casting method. The thin metal layers collapse and the hologram is destroyed when attempts are made to remove it a security.
From Research Disclosure, December 1995/787, it is known that markings ranging from light to invisible to the eye may be made from suitable polythiophenes, e.g. 3,4-polyethylenedioxythiophene. They absorb light close to the infrared range whereas in visible light they are substantially transparent. A further possibility of distinguishing polythiophene markings is based on their electrical conductivity. Differences in their surface resistance between the non-conductive support and the conductive markings may be registered by suitable arrangements of electrodes.
Furthermore, it is known from European patent specification EP 753 623 to make a security sheet incorporating an electrically conductive element as protection against counterfeiting. The conductive element consists of a polythiophene polymer. Printing dyes containing metallic pigments are similarly mentioned.
Finally, German patent specification DE 38 43 075 describes an arrangement in a security document with a security thread containing a metallic coating. Immediately adjacent the coating there is provided a layer containing electrically conductive pigments or a layer of plastic rendered electrically conductive by its molecular structure. The layer serves to bridge possible interruptions in the metallic coating of the security thread.
The reason for using the holograms as security attributes for individual articles, such as securities or goods, is predicated on their fabrication in large numbers in mostly large-scale industrial plants at as low a price as possible. This is, however, limited by currently practiced technological fabrication processes.